Is upgrading custombuild 1.2 to 2.0 safe?

Ive all machines on custombuild 2.0 and all is great. Recently got +1 machine to care of and it has custombuild 1.2 and Debian 7. Would like to switch to custombuild 2.0, but never did this sort of thing with DSA.

The installation process seems easy but can it do harm to services? I cannot to clone it and test it on other machine unfortunatelly.

THe server is on production, has few clients websites (classic stuff). I dont care if PHP would broke some websites - It will be fixed with clients.
I care if something can go wrong from DA perspective (and DA or especially exim/dovecot wont work).

Thanks for any information.